The present invention relates to techniques for preparing a fuel-air mixture for combustion in an engine.
A present thrust of gas-turbine engine technology seeks to attain reduced emissions of nitrogen (NOx) and hydrocarbon compounds. Prior-art techniques for accomplishing such reduced emissions almost invariably result in reduced thermodynamic efficiency or substantially increased capital costs.
NOx compounds are produced by reaction of the nitrogen in the air at elevated temperatures conventionally found in the combustors of a gas turbine engine. NOx formation can be reduced by reducing the maximum flame temperature in the combustor. Injection of steam into the combustor reduces the maximum flame temperature in the combustor at the cost of thermodynamic efficiency. Penalties must also be paid in water use, and water treatment capital and operating costs. The amount of steam injection, and its attendant costs, rises with the amount of NOx reduction desired. Some states and foreign countries have announced targets for NOx reduction that infer such large quantities of steam that this solution appears less desirable for future systems.
NOx compounds can be removed from the exhaust downstream of a gas turbine engine by mixing a reagent such as, for example, ammonia, with the exhaust stream and passing the resulting mixture through a catalyst before venting to the atmosphere. The catalyst encourages the reaction of the NOx compounds with the reagent to produce harmless components. This technique, although successful in reducing NOx compounds to target levels, requires substantial additional capital outlay for the catalyst bed, a larger exhaust system to provide room for the large catalyst bed and spray bars to deliver the reagent into the exhaust stream. The on-going cost of large quantities of the reagent must also be borne.
The maximum flame temperature can be reduced without steam injection using catalytically supported combustion techniques. A fuel-air mixture is passed through a porous catalyst within the combustor. The catalyst permits complete combustion to take place at temperatures low enough to avoid NOx formation. Several U.S. patents such as, for example, U.S. Pat. No. 4,534,165 and 4,047,877, illustrate combustors having catalytically supported combustion.
Reduction or elimination of hydrocarbon emissions is attainable by ensuring complete combustion of the fuel in the combustor. Complete combustion requires a lean fuel-air mixture. As the fuel-air mixture is made leaner, a point is reached at which combustion can no longer be supported. The presence of a catalyst also permits combustion of leaner fuel-air mixtures than is possible without the catalyst. In this way, catalytically supported combustion aids in reducing both types of environmental pollution.
A critical problem, not completely solved by the referenced prior-art patents, is attaining a uniform flow field of fuel-air mixture across the entire face of a catalyst bed. That is, the fuel-air mixture and the gas velocity vary across the face of the catalyst bed, resulting in uneven combustion across the catalyst. This reduces combustor efficiency and can permit unburned hydrocarbons to escape to the exhaust.
In the referenced '877 patent, for example, liquid fuel and air are injected into a chamber upstream of the catalyst bed. The fuel-air mixture then flows through the catalyst bed, wherein the fuel and air react. As pointed out in this patent, unburned fuel may exit the catalyst. A gas-fuel burner downstream of the catalyst is relied on to burn this unburned liquid fuel.
The '165 patent breaks up the catalytic bed into concentric zones, each having its own liquid fuel and air supply. Although the patent proposes that the advantage of breaking the catalytic bed and fuel-air supply into zones is found in the resulting ability to stage fuel to the individual zones, it might be presumed that the resulting smaller area of catalytic bed fed by each fuel-air supply device may improve the uniformity of fuel-air mixture reaching an enabled zone of the catalytic bed.
Further improvement in flow-field uniformity is reported in an article entitled "Performance of a Multiple-Venturi Fuel-Air Preparation system" by Robert Tacina published in NASA Conference Publication No. 207 held on Jan. 9 and 10, 1979 "Pre-Mixed, Pre-Vaporized Combustion Technology Forum". This article, a copy of which accompanies the present patent application, discloses a plurality of parallel venturi tubes disposed across the flow path of air leading to a catalyst bed. A vaporized liquid fuel is injected into the inlet of each venturi. In flowing through a venturi tube, the air and fuel are thoroughly mixed. The mixtures exiting all of the venturi tubes further mix together downstream of the venturi tubes to produce a flow field that is substantially uniform in velocity and fuel-air mixture across the downstream gas mixture.
The multiple-venturi tube device disclosed at the NASA conference has several drawbacks that make it unsuitable for applications envisaged for the present invention. First, the multiple venturi tubes are machined out of a single piece of metal. This is a costly way to form such a structure, and results in feather edges at the exits which may not endure in the severe operating enviroment of a gas turbine combustion system. Second, each venturi tube is fed liquid fuel through an individual tube of small diameter. It is foreseen that such small tubes can become clogged, rendering the affected venturi tubes inoperative. In a large device, the large number of such tubes is a reliability problem.
In addition to addressomg the above drawbacks of the referenced multiple-venturi device, it is an objective of the present invention to replace liquid fuel with fuel gas. In the start-up procedure of a catalytic reactor, external heat is required until the catalytic reactor attains an operating temperature. One way of providing the external heat includes a preburner disposed upstream of the multiple-venturi tube assembly. It is believed that injecting a fuel gas into the inlet of the venturi tubes could lead to flash back to the preburner or flame holding at the inlets of the venturi tubes. Neither of these effects is desirable.